WASHINGTON
— Thursday night's presidential debate, likely to be watched by some 50 million Americans, could change the course of the entire campaign. But for some voters at least, the outcome will be irrelevant, as they've already cast their ballots.

In a gradual but significant change to the political process, many more states are now allowing people to vote ahead of Election Day, without having to provide the usual excuses. The idea is that, in an age when many Americans are juggling demands of work and family and are increasingly strapped for time, more convenient forms of voting may help stem a decline in turnout.

This year, 35 states are allowing unconditional forms of early voting, either on a voting machine or in person by absentee ballot, according to Electionline.org, while 24 states are allowing no-excuse absentee balloting. Voting has already begun in Iowa and Maine, and will start in a number of other states over the next few weeks.

Experts estimate that at least a quarter of the total national vote may be cast before Nov. 2. The trend is producing major changes in how campaigns operate, with get-out-the-vote drives that last weeks instead of days and are ever more individually targeted. Parties can not only track voters who have requested early ballots, but in many cases, can hand-deliver those ballots - a factor that has some experts worried about the potential for fraud.

More important, the trend is likely to have a significant, if not yet entirely understood, effect on the timing and trajectory of campaigns. Attacks launched in August or September - such as the slew of third-party ads targeting Sen. John Kerry's war record - may prove more deadly than any barbs that come at the end. It also could affect the calculus of when to peak: Senator Kerry's much-vaunted tendency to pick up steam in the final weeks of a race, if it happens, could now come too late to make a difference.

With many of the most important moments of the campaign, such as debates, still to come - not to mention unpredictable external events, from violence in Iraq to changes in oil prices to the looming possibility of a terror attack - voters could easily wind up making their decisions based on entirely different sets of facts and conditions, depending on when they mark their ballots.

"It changes the dynamic in the course of the election," says Tova Wang of the Century Foundation in New York. "At best, [early voters] are losing out on possible details on the character and policies of the candidates." At worst, she adds, voters could be "completely disenfranchised" - as in 2002, when Minnesotans who voted early for Sen. Paul Wellstone found their ballots irrelevant after he was killed in a plane crash.

Both parties recognize the stakes - and are putting enormous resources into marshalling votes early. Republicans historically have garnered a greater share of absentee votes overall. But the tallies in individual states has varied: In 2000, Al Gore's advantage among absentee ballots in Iowa allowed him to win the state, despite George W. Bush's winning a greater share of the vote at the polls.

Proponents of early voting argue that it actually gives people a chance to cast a more informed vote, letting them go about it on their own extended timetable. With complicated ballot initiatives and other measures proliferating - and some voter guides approaching the length of phone books - voters can take more time going through their ballots and getting the specific facts they need to make a decision.

"With early voting, you can have a more informed voter, who has more time to understand what's on the ballot," says Brian Lundy, founder of helpingamericansvote.org, a nonpartisan corporation.

Making voting seem more manageable - and more convenient - proponents add, could boost participation among voters who might otherwise decide they don't have enough time to study up on the issues or get to a polling place on Election Day. But a recent study by the Committee for the Study of the American Electorate found that, in recent elections, early voting has actually hurt turnout in states that have offered that option.

Most early voters tend to be strong partisans - and therefore people who would be most likely to vote, anyway. To the parties, getting those votes "in the bank" early helps them gain a better sense of where they stand, and frees volunteers to concentrate more of their efforts on soft supporters in the final days.

That could have a subtle impact on the shape of the campaigns' messages, too, allowing them to shift from focusing on turning out their bases to appealing to more elusive swing voters.

"The party that best figures out how to take advantage of the new rhythm is the one that's going to be most successful at early voting," says Dan Seligson, editor at Electionline.org. "It simplifies the campaign quite a bit to have these votes already in hand. Then you focus the rest of your efforts on the day of, or immediately before."

But spreading their get-out-the-vote drives over five or six weeks could also dilute the impact of those efforts, in contrast to unleashing more resources for a final 72-hour blitz.

And some analysts speculate that having fewer strong partisans head to polls on Election Day saps a sense of excitement and community spirit that might otherwise pull less enthusiastic voters to the polls.

"Look at who is being urged to do early voting: it's the partisans that the parties have identified as their strong supporters," says Rob Richie of the Center for Voting and Democracy. "[But] if you take reliable voters out of the Election Day pool of voters, they won't pull in, in this communitarian sense, those who may be less enthusiastic about voting."